Proper tire pressure on any wheeled vehicle is important from both a safety and an operating cost perspective. With regard to aircraft, especially large commercial airplanes, the weight of such vehicles can affect tire pressure. Low pressure leads to tire over deflection and heat build-up within the tire, which can result in premature or excessive wear. The heavier the vehicle, when supported by an under pressurized tire, the greater the premature wear, such as ply separation, uneven wearing of the tread, or rapid wearing of the shoulder. Alternatively, over inflating a tire may cause excessive wearing of the tread center, reduced braking efficiency, abnormal tire growth, and may make the tread more susceptible to cutting by foreign objects. Regardless of whether the tire is over inflated or under inflated, the negatives associated with each translate directly into increased maintenance cost.
Another problem with heavy vehicles, such as commercial aircraft, is that heat build-up in the tires and wheel rims occurs rapidly when such vehicles are moving even short distances and especially after landing. This heat build-up increases tire pressure and is slow to dissipate thus making it difficult to accurately measure the proper tire pressure within short windows of time. To properly determine tire pressure the measurement should be taken only when the internal tire gas temperature is equal to the ambient temperature. This is considered a “cold tire” pressure. The heat generated by rolling large heavy wheeled vehicles can increase tire gas temperatures to such levels that it may require 3-4 hours for the tire, the rim, and the internal tire gas temperature to equilibrate with the external ambient air temperature.
Further complicating this temperature difference between the ambient outside air temperature and the tire gas temperature is that aircraft operators prefer flexibility in timing tire pressure checks. Some operators find it difficult or too inconvenient to measure tire pressure immediately after an overnight or extended park time when tires have reached the preferred equilibrium with the outside ambient temperature. As such, airline maintenance personnel experience difficulty ensuring tires are at the proper and recommended pressure. No known existing tire pressure indication system takes into account a procedure for verifying that the tire is at the cold tire pressure temperature before a measurement is taken. Likewise, no known tire pressure monitoring system considers or calculates a stationary or park time of the vehicle to ensure that the temperature of the tire gas necessarily equals the ambient temperature. Although some systems attempt to measure tire gas temperatures and make a temperature adjustment during maintenance using thermocouples in the wheel structure, these systems are flawed because tire temperature is non-uniform during and immediately following braking and rolling.
Yet another concern arises when an aircraft takes off for a destination where ambient temperatures are significantly lower (≧25° C.) from those of the airport of departure. The minimum required cold pressure must be maintained for the cooler climate. Without an accurate determination of cold tire pressure at the departure airport it is very difficult, if not impossible, to adjust the tire pressure before departure so that the tires are correctly inflated at the required or recommended cold tire pressure for the ambient colder conditions at the destination airport or at the coldest final destination airport.
Accordingly, there is a need to develop a tire pressure monitoring system that takes into account the above-mentioned concerns. The system should be capable of determining accurate tire pressures and to recommend corrective action to either inflate or deflate a tire not at the recommended cold tire pressure or at an extreme temperature for a destination location. Such a system would significantly reduce airline operating costs due to improved tire life and improve safety.